Thunderbird also has problems with folders deleted by other clients and folders with lots of messages (it couldn't open a folder with 243,000 messages, but Outlook XP could). But generally I find it fast and very usable, although not as good as Outlook 2003 when it comes to caching.

You can limit the number of connections Mozilla will cache, in the advanced imap server prefs settings, so it's under your control. Currently, we limit the minimum number of connections to 2, so we'll be able to have a connection open for the inbox biff, but I'll probably relax that so that if you really only want one connection made, you can do that.

I've checked in support for the IMAP Idle command, <http://bugzilla.mozilla.org/show_bug.cgi?id=141369> - it can be disabled on a per-server basis by setting the server pref "use_idle" to false. This will be in tomorrow's nightly trunk builds. Please let me know if there are any problems.